1. Field of the Invention
The present invention relates to an electro-luminescence display (ELD) device, and more particularly, to an electro-luminescence display device and a driving method thereof that prevents a rise in a threshold voltage of a driving thin film transistor at each pixel and provides stable display brightness.
2. Discussion of the Related Art
Many efforts have been made to research and develop various flat display devices, such as liquid crystal display (LCD) devices, field emission display (FED) devices, plasma display panel (PDP) devices, and electro-luminescence (EL) display devices, as a substitute for cathode ray tube (CRT) devices. These flat display devices have advantageous characteristics of thin profile, lightness, and compact size. In addition, an electro-luminescence (EL) display device has another advantage in that it is a self-luminous type display capable of emitting light using a phosphorous material.
An EL display device generally is classified as an inorganic EL device if the phosphorous material includes an inorganic material or is classified as an organic EL device if the phosphorous material includes an organic compound. In general, an organic EL device includes an electron injection layer, an electron carrier layer, a light-emitting layer, a hole carrier layer and a hole injection layer disposed between a cathode and an anode. When a predetermined voltage is applied between the anode and the cathode, electrons produced from the cathode are moved, via the electron injection layer and the electron carrier layer, into the light-emitting layer, while holes produced from the anode are moved, via the hole injection layer and the hole carrier layer, into the light-emitting layer. Thus, the electrons and the holes fed from the electron carrier layer and the hole carrier layer are re-combined at the light-emitting layer, thereby emitting light.
The organic ELD generally is manufactured using a relatively simple process including a deposition process and an encapsulation process. Thus, an organic ELD has a low production cost. Further, the organic ELD can operate using a low DC voltage, thereby having a low power consumption and a fast response time. The organic ELD also has a wide viewing angle and a high image contrast. Moreover, since the organic ELD is an integrated device, the organic ELD has high endurance from external impacts and a wide range of applications.
A passive matrix type ELD that does not have a switching element has been widely used. In the passive matrix type ELD, scan lines intersect signal lines defining a plurality of pixels in a matrix-arrangement, and the scan lines are sequentially driven to excite each of the pixels. However, to achieve a required mean luminescence, a moment luminance needs to be as high as the luminance obtained by multiplying the mean luminescence by the number of lines.
There also exists an active matrix type ELD, which includes thin film transistors as switching elements within each pixel. The voltage applied to the pixels are charged in a storage capacitor Cst so that the voltage can be applied until the next frame signal is applied, thereby continuously driving the organic ELD regardless of the number of gate lines until a picture of images is finished. Accordingly, the active matrix type ELD provides uniform luminescence, even when a low current is applied.
FIG. 1 is a schematic block diagram illustrating an active matrix type electro-luminescence display device according to the related art. In FIG. 1, an active matrix type EL display device includes an EL panel 20 having pixels 28 arranged at intersections between gate lines GL and data lines DL, a gate driver 22 for driving the gate lines GL, and a data driver 24 for driving the data lines DL. The gate driver 22 sequentially applies a scanning pulse to the gate lines GL to drive the gate lines GL. In addition, the data driver 24 converts digital data signals inputted from an exterior source to analog data signals and applies the analog data signals to the data lines DL whenever the scanning pulse is supplied. Each of the pixels 28 receives the data signal from a respective one of the data lines DL when the scanning pulse is applied to a corresponding one of the gate lines GL, to thereby generate light corresponding to the data signal.
FIG. 2 is a detailed circuit diagram illustrating a pixel of the electro-luminescence display device shown in FIG. 1. As shown in FIG. 2, each of the pixels 28 includes an EL cell OEL having an anode connected to a supply voltage source VDD and a cathode connected to a cell driver 30. The cell driver 30 also is connected to the respective gate line GL, the respective data line DL and a ground voltage source GND to drive the EL cell OEL.
In addition, the cell driver 30 includes a switching thin film transistor T1, a driving thin film transistor T2, and a storage capacitor Cst. The switching thin film transistor T1 includes a gate terminal connected to the respective gate line GL, a source terminal connected to the respective data line DL, and a drain terminal connected to a first node N1. The driving thin film transistor T2 includes a gate terminal connected to the first node N1, a source terminal connected to the ground voltage source GND, and a drain terminal connected to the EL cell OEL. The storage capacitor Cst is connected between the ground voltage source GND and the first node N1.
Further, the switching thin film transistor T1 is turned ON, when a scanning pulse is applied to the respective gate line GL. When the switching thin film transistor T1 is turned ON, it applies the data signal supplied to the respective data line DL to the first node N1. Then, the data signal supplied to the first node N1 is charged into the storage capacitor Cst and applied to the gate terminal of the driving thin film transistor T2. The driving thin film transistor T2 controls a current amount I fed, via the EL cell OEL, from the supply voltage source VDD in response to the data signal, to thereby control a light-emission amount of the EL cell OEL.
Moreover, the driving thin film transistor T2 can keep a turn-ON state by the data signal charged in the storage capacitor Cst even though the switching thin film transistor T1 is turned OFF, and can still control a current amount I fed, via the EL cell OEL, from the supply voltage source VDD until a data signal at the next frame is applied. In this case, the current amount I flowing the EL cell OEL can be expressed as the following equation:
                    I        =                              W                          2              ⁢              L                                ⁢                                    Cox              ⁡                              (                                  Vg2                  -                  Vth                                )                                      2                                              (        1        )            “W” represents a width of the driving thin film transistor T2, and “L” represents a length of the driving thin film transistor T2. Further, “Cox” represents a value of a capacitor provided by an insulating film forming a single layer when the driving thin film transistor T2 is manufactured. Also, “Vg2” represents a voltage value of a data signal inputted to the gate terminal of the driving thin film transistor T2, and “Vth” represents a threshold voltage value of the driving thin film transistor T2.
In the above equation (1), “W,” “L,” “Cox” and “Vg2” are constantly maintained irrespectively of a lapse of time. However, the threshold voltage value “Vth” of the driving thin film transistor T2 deteriorates with the lapse of time.
In particular, a positive (+) voltage is continuously supplied to the gate terminal of the driving thin film transistor T2. Specifically, the continuously applied positive voltage causes the threshold voltage Vth of the driving thin film transistor T2 to be increased with a lapse of time. In addition, as the threshold voltage Vth of the driving thin film transistor T2 increases, a current amount flowing through the EL cell OEL is reduced, thereby decreasing an image brightness and deteriorating an image quality.
FIGS. 3A and 3B are diagrams illustrating atomic arrangements of amorphous silicon, and FIG. 4 is a graph illustrating a deterioration of a driving thin film transistor of the pixel shown in FIG. 2. The driving thin film transistor T2 (shown in FIG. 2) is made from hydride amorphous silicon. Hydride amorphous silicon can be easily made in a large dimension and can be deposited on a substrate at a low temperature of less than 350° C. Thus, a majority of thin film transistors have been made using hydride amorphous silicon.
However, as shown in FIG. 3A, hydride amorphous silicon has an irregular atomic arrangement having a weak/dangling Si—Si bond 32. As shown in FIG. 3B, with the lapse of time, Si breaks from the weak bond, and electrons or holes are re-combined at the atom-departed place. Since an energy level is changed due to a variation in the atom arrangement of the hydride amorphous silicon, the threshold voltage Vth of the driving thin film transistor T2 is increased gradually into Vth′, Vth″ and Vth′″ as shown in FIG. 4 with the lapse of time.
Accordingly, the image brightness of the electro-luminescence display device according to the related art degrades over time because the threshold voltage Vth of the driving thin film transistor T2 is increased to Vth′, Vth″ or Vth′″ with the lapse of time. In addition, since a partial brightness reduction of the EL panel 20 produces a residual image, thereby seriously deteriorating an image quality.